Femtocell base stations are known in different communication standards. The 3GPP standard with specification number 3GPP TS 25.467 v 10.2.0 for instance describes the UTRAN architecture for a femtocell base station for a 3G Home Node B (HNB).
Specifically, the 3GPP TS 25.467 standard specification describes a UE registration function as “The UE Registration Function for HNB provides means for the HNB to convey UE identification data to the HNB-GW in order to perform access control or membership verification for the UE in the HNB GW. The UE Registration also informs the HNB-GW of the specific HNB where the UE is located.”
According to the standard, UE registration is performed by the HNB and the HNB registers a specific UE with the HNB-GW. The registration is triggered when the UE attempts to access the HNB via an initial NAS message over the Uu interface (e.g., Locating Updating (LU) Request or a CM Service Request initial NAS message) and there is no context in the HNB allocated for that UE.
Chapter 5.1.2 of the same standard describes the case where a HNB registers a UE in the HNB GW when either the UE or the HNB, or both, do not belong to a CSG. In this case, the standard describes the option that the HNB may in certain cases fetch the permanent identity of the subscriber with a NAS Identity Request procedure before registering the subscriber in the HNB GW. The step of fetching the permanent subscriber identity consists of a NAS Identity Request message sent by the HNB, and a NAS Identity Response message sent by the UE as a reply, containing the subscriber identity (IMSI).
A problem with the existing standard as described above is that the HNB uses a NAS message to fetch the identity of the subscriber. However, NAS messages are defined to be exchanged between the CN and the UE, and they are standardized in 3GPP TS 24.008. The NAS messages contain a ‘sequence number’, N(SD), that allows the CN to detect duplicated NAS messages sent by the UE. NAS messages may be duplicated by the Data Link layer in some cases, for example at channel change before the last layer 2 frame has been acknowledged by the peer. The NAS sequence number mechanism is described in 3GPP TS 24.007, chapter 11.2.3.2.3 and subchapters. The first NAS message is sent by the UE with sequence number ‘0’. The CN will receive this first NAS message with sequence number ‘0’ in the RANAP Initial UE message. However, when the HNB fetches the subscriber identity with a NAS Identity Request message, then the UE will step the sequence number and reply with a NAS Identity Response with sequence number ‘1’. This means that the CN will discard any further NAS message sent by the UE to the CN, since the UE will send the next NAS message to the CN with the sequence number ‘2’, but the CN expects the next NAS message to have the sequence number ‘1’ according to TS 24.007. Thus, e.g. the Location Update Request or the CM Service Request in the CN will fail in certain cases, for example if the CN tries to authenticate the subscriber with a NAS Authentication Request, since the CN will discard the Authentication Response from the UE.
WO 2007/136339 discloses a method and radio network controller (RNC) in a radio access network (RAN) for controlling access to cellular telecommunication system. Upon receiving an access request from a given user equipment (UE) through a given radio base station (RBS), the RNC retrieves authorization information from an access control database within the RAN. The authorization information indicates whether the given UE is authorized to access the system through the given RBS. The RNC alternatively grants access or denies access to the UE based on the retrieved authorization information. The RBS is particularly useful for controlling access through small cells with limited capacity.
According to WO 2007/136339 the RAN is injecting NAS messages in the communication between the UE and the CN, whereby there is the possibility that the send sequence number, N(SD), may get out of synchronization between the UE and the CN. This may be a problem if the CN is performing any checks on the received N(SD). Thus, a procedure is performed to synchronize the N(SD). If the CN is performing checks on the N(SD), the Femto-RNC can be configured to perform N(SD)-mapping during the location updating procedure when the identification procedure was triggered from the Femto-RNC. The N(SD)-mapping procedure may include the steps:
a) The initial LOCATION UPDATING REQUEST message from the UE with N(SD)=0 is forwarded to the CN.
b) The N(SD) for all uplink NAS-messages is decreased by one, also taking into account the maximum value of N(SD) depending on the indicated core network release.